1. Field of the Invention
The present invention relates to an encoder circuit for converting a color-difference signal into a carrier chrominance signal.
2. Description of the Prior Art
Recently, considerable development has been achieved in improvements in picture quality of video tape recorders (hereinafter to be briefly called VTR), and now, component VTRs, which record and play back component signals separated from a composite video signal, are prevalently used for broadcasting and industrial applications. However, composite video signals are frequently used in peripheral equipment, and therefore, there is the need for signal conversion between such equipment and the component VTRs. Conventionally, the signal processing circuit as the encoder circuit for converting the component signals into the composite signal has been constructed as shown in FIG. 9. Color difference signals, R-Y signal and B-Y signal, for example, inputted to terminals 20, 24 are delivered through clamping circuits 21, 25 to balanced modulators (hereinafter to be briefly called BM) 22, 27, for modulating color subcarriers (3.579545 MHz in the NTSC system), which are generated by a color subcarrier generator 23 and a 90.degree. shifter 26 so as to have phase angles 90.degree. apart from each other, and the modulated signals are then mixed by a mixer 28 to be outputted from a terminal 29.
With this construction, however, the input signals would not be completely clamped by the clamping circuits 21, 25 due to malfunction of the clamping circuits or due to temperature characteristics of the clamping circuits or the BMs. That is, even if the color difference signal is at zero level, the color subcarrier would leak to be outputted, i.e., a so-called "carrier leak" would occur to cause changes in hue and saturation of the output chrominance signal thereby to greatly deteriorate the picture quality.